Fix download-boxes.sh if no boxes are present (techno-tim#106)

In case of grep not matching any line, it would return an error code
and thus stop the script. This patch sets "present_boxes" to an empty
value in case any of the commands fail.

.github/download-boxes.sh

@@ -17,11 +17,14 @@ all_boxes=$(cat "${GIT_ROOT}"/molecule/*/molecule.yml |
  uniq)
 
 # Read the boxes that are currently present on the system (for the current provider)
-present_boxes=$(vagrant box list |
- grep "${PROVIDER}" | # Filter by boxes available for the current provider
- awk '{print $1;}' | # The box name is the first word in each line
- sort |
- uniq)
+present_boxes=$(
+ (vagrant box list |
+ grep "${PROVIDER}" | # Filter by boxes available for the current provider
+ awk '{print $1;}' | # The box name is the first word in each line
+ sort |
+ uniq) ||
+ echo "" # In case any of these commands fails, just use an empty list
+)
 
 # The boxes that we need to download are the ones present in $all_boxes, but not $present_boxes.
 download_boxes=$(comm -2 -3 <(echo "${all_boxes}") <(echo "${present_boxes}"))